The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
DL downlink
DCH dedicated channel
DRX discontinuous reception
E-UTRAN evolved UTRAN (also termed LTE)
FACH forward access channel
GERAN GSM-enhanced data rates for global evolution (EDGE)
GSM global system for mobile communications
HSPA high speed packet access
LTE long term evolution
PCH paging channel
RAT radio access technology
RRC radio resource control
TDD time division duplex
UE user equipment
UMTS universal mobile telecommunications system
UL uplink
URA user registration area
UTRA UMTS terrestrial radio access
UTRAN UTRA network
WCDMA wideband code division multiple access
The exemplary embodiments detailed herein are in the context of the UTRAN (e.g., WCDMA, HSPA, and GERAN) wireless systems by which a UE attached to a UTRAN cell via a FACH or DCH (the CELL-FACH and CELL-DCH states respectively) seeks to transition and attach to another higher priority cell (e.g., an inter-RAT E-UTRAN/LTE cell or an inter-frequency UTRAN cell operating on a higher priority frequency layer). These systems are exemplary only and these teachings are not limited only to those wireless systems but are more generally applicable.
There is intentionally no specification in LTE Release 8 for detailing how a UE in the CELL-FACH state may reselect to the LTE system since at the time it was assumed that such a UE could transition to the idle/cell-PCH/URA-PCH state to make the re-selection. Since then a problem has been identified (i.e., in 3GPP RAN2#73 and RAN4#58 meetings) in which a UE may not be in an idle/Cell-PCH/URA-PCH state long enough to measure, analyze, and reselect from UTRAN to a higher priority E-UTRAN cell (e.g. LTE hotspots). Specifically, reselection from the UTRAN CELL-FACH state when under certain network configured state transition timers (e.g., the timer used for transitioning from CELL-FACH to URA-PCH) and traffic patterns, combined with the specified UE measuring rules, may make it impossible in certain circumstances for a UE to ever complete a reselection to an E-UTRAN cell. These conditions arise when the traffic pattern is “bursty” (e.g., when the UE is a packet switched oriented device used for always online type services, such as a combined UMTS/LTE USB (universal serial bus) modem which can be connected to a laptop).
One possible solution discussed was to enable reselection to an LTE cell while the UE was in the CELL-FACH state. This may not be universally suitable; it is a network equipment choice whether to use measurement occasions in the CELL-FACH state even if the same network operator deploys LTE cells in the same geographic area as its UTRAN cells.
It is therefore desirable to also have solutions for the UE to use when making frequent transitions between UTRAN CELL-FACH and CELL-PCH or URA-PCH states which are not dependent on the UTRAN's ability to configure and accept a UE's measurements in the CELL-FACH state.
In the current specification, the UE is required to perform the following steps:                a. Stop monitoring LTE neighbor cells when moving from Idle or PCH states to CELL-FACH state.        b. Start monitoring LTE neighbor cells when leaving CELL-FACH state to Idle or PCH states. The combination of steps a) and b) implies the UE needs to re-start its measurement evaluation and discard measurements taken before moving to the CELL-FACH state when it starts a new measurement after leaving the CELL-FACH state.        c. Measure high priority frequency layers at least every 60 s per configured higher priority frequency layer. This implies some UEs may wait 60 seconds per configured higher priority frequency layer before making a measurement after entering idle or PCH states.        d. Measure high priority (LTE) frequency layers at minimum each Kcarrier * Tmeasure,EUTRA. For a typical 1.28 DRX cycle this is 6.4 seconds if one E-UTRA higher priority carrier is configured.        e. Measure and average at least 2 samples before making the reselection decision, and these measurements must be separated by at least (Kcarrier * Tmeasure,EUTRA)/2. Even if one assumes the UE takes its first measurement immediately after moving out of the CELL-FACH state, at least 2.5 * DRX cycles will pass before a decision can be made and reselection can be performed. Typically this will be at least 3.84 seconds total for a 1.28 s DRX cycle. Taking into account that it is undesirable for battery life for the UE to wake up during the DRX cycle, the earliest time to transition to an LTE neighbor cell is 3 DRX cycles after the UE moves out of the Cell-FACH state.        f. Apply Treselection before moving to the new cell. Treselection is a delay imposed so the UE can check that the serving cell does not recover from giving bad coverage before the UE completes the reselection, and also allows the UE to check that the neighbor/new cell remains best ranked. This check deliberately adds additional delay to the reselection.        g. If, after detecting a cell in a higher priority search, it is determined that reselection has not occurred then the UE is not required to continuously measure the detected cell to evaluate the ongoing possibility of reselection and may wait for 60 s per configured higher priority frequency layer before evaluating the frequency again.        
Co-owned US U.S. patent application Ser. No. 13/023,675 entitled PRIORITY MEASUREMENT RULES FOR CHANNEL MEASUREMENT OCCASIONS (filed Feb. 9, 2011) details the use of measurement occasions in the CELL-FACH state to enable reselection. The solution detailed by the exemplary embodiments herein is even more universally adoptable since it is anticipated to be operable for all network configurations/timer settings.